Halftone printing techniques enable continuous tone images to be represented on a bi-level device, such as an inkjet printer, by simulating the screening process used in conventional printing. Some halftone techniques model continuous tone pixels with halftone cells. A halftone cell includes a number of points, each of which may be turned on or off. Various halftone dot patterns can be created by selectively turning particular points on. For example, as more points are turned on, the halftone cell appears more intense. Thus, less intense pixel values can be modeled by halftone cells that have more points turned off, and more intense pixel values can be modeled by halftone cells that have more points turned on. Various halftone modeling algorithms can be used to control whether points are turned on or off, depending on the continuous tone pixel value being modeled.
A halftone image file can be viewed as a bit map. Each bit is used to indicate whether a point is on or off. If halftone cells are used to create a halftone bit map, each cell defines a number of bits in the bit map. Halftone bit maps, however, can be created without generating the halftone cells that correspond to the continuous tone pixels. Indeed, halftone screening processes have been developed in which a continuous tone image file is converted into a halftone bit map without generating the individual halftone cells.
Halftone printing devices typically print dots of ink according to the halftone bit map. For example, if the point in the bit map is on, the printing device prints a dot in the area associated with the point, and if the point in the bit map is turned off, the printing device does not print a dot in the area associated with the point. Color halftone printing devices make use of different channels, e.g., Cyan (C), Magenta (M), Yellow (Y) and Black (K), to define colors. Thus, color halftone printing devices may use a plurality of monochromatic bit maps. Each monochromatic bit map defines the presence or absence of dots for a particular channel.
The visual appearance of halftone prints can be negatively impacted by overlap of ink in adjacent dots. For example, ink dots printed by a halftone printing device can smear or wick on the printing substrate. Moreover, if too much ink is applied, the printing substrate can be oversaturated, possibly causing the substrate to buckle or tear. In either case, the halftone print may appear visually incorrect.